Initially, it is noted that Institute of Electrical and Electronics Engineers (IEEE) Standard 802.11-2016 is used as the base reference for disclosures used in this disclosure, the entire contents of which are incorporated herein by reference. The IEEE 802.11 Standard is commonly referred to as “Wi-Fi”. A Wi-Fi network generally includes an access point (AP) and a number of stations (STA).
Determination of the location of WLAN devices can be performed by various means. Of particular interest for this disclosure is the location of devices that are based upon the IEEE 802.11 technology, commonly known as Wi-Fi. Various methods can be used to locate an access point (AP), or a station (STA). These methods may be classified as active, passive and combined active and passive. In an active location scheme, a device that is determining the range and location, the measuring device, transmits certain packets to the device being located, the target device, and the common method is to measure the time of arrival (TOA) of the response from the target device and compare that to the time of departure (TOD) that the packet was transmitted by the measuring device.
In such location systems it is common to use multiple measuring devices to determine the device location. In such a scheme, simultaneous TOA and/or TOD measurements are taken by different measuring devices situated at different points and the location of the target device calculated.
In an active location scheme, TOD may be measured for a packet that is transmitted from the measuring station addressed to the target station. The TOA of the response from the target station at the measuring station is then also measured. If the turnaround time for the target station to receive the packet from the measuring station and to start to transmit the response is known, then the time difference at the measuring station between the TOA and the TOD, minus the turnaround time at the target station will be directly proportional to twice the distance of the target station from the measuring station. For example, if the target station is a wireless device based upon IEEE 802.11 technology, and if the packet transmitted from the measuring station to the target station is a data packet, the response from the target station will normally be an acknowledgement (ACK) packet. If the packet transmitted from the measuring station to the target station is a control packet, for example, a “ready to send”, also referred to as a “request to send”, (RTS) packet, then the response from the target station will normally be a clear-to-send (CTS) packet. In these two examples, the turnaround time at the target station is defined in the IEEE 802.11 standard as the short interframe spacing (SIFS) which is a preset value. Hence, the time delay δt, between the measuring station and the target station, may be determined from the calculation δt=(TOA−TOD−SIFS)/2 and the distance between the measuring station and the target station is then δt/c, where c is the speed of light. This method of estimating the distance to a target station by measuring the TOD and TOA and accounting for the turnaround time is known in the art.
FIG. 1 is a diagram of a typical location system 100 which includes three measuring stations 10a, 10b and 10c (referred to collectively herein as “measuring stations” or “measuring receivers” 10). The target 12 is a wireless device, such as, for example, an Access Point (AP) that is to be located by the three measuring stations 110. The distance of the AP 120 from measuring station 10a is D1, 130. The distance of the AP 120 from measuring station 10b is D2, 140. The distance of the target 12 from measuring station 10c is D3, 150. The time delay, δt1, determined from the calculation δt=(TOA−TOD−SIFS)/2), is measured for transmissions from measuring station 10a and this can be used to calculate the distance D1 130 using the formula D1=δt1/c where c is the speed of light. Similarly δt2 and δt3 measurements result in the determination of distances D2 140 and D3 150. The methods for calculating the location of AP 120 using the distances D1, 130, D2 140 and D3 150 are well known in the art and are beyond the scope of this disclosure.